The present invention relates to fuel injection systems for internal combustion engines, in particular, systems suitable for injection of low viscosity fuels such as Di-Methyl Ether (DME) into compression-ignition engines.
Some low-viscosity fuel injection systems have been designed following the principle of the known high-pressure common rail systems for diesel fuel, in which fuel is pressurized by a single pump into a pressure vessel or rail that is common for a plurality of injectors, and injections are controlled by electrically actuated valves placed between the common rail and the injectors. This kind of injection system is better suited for fuels with widely variable, pressure and temperature-dependent properties typical to these of DME, than injection systems based on the pump-line-nozzle or unit injector principle, partially because the processes of creating and controlling the injection energy for each injection are divided in time and therefore do not complicate each other as much as when they occur simultaneously. The subject of the present invention is a common rail fuel injection system for injecting a low viscosity fuel into internal combustion engines.
An example of a prior art injection system is disclosed in the U.S. Pat. No. 6,189,517.
One technical difficulty that arises when a common rail injection system is designed for use with low-viscosity fuels is that, due to relatively low specific energy of low viscosity fuels such as DME, the volume to be passed through the injectors and control valves to obtain a given engine power, is relatively big. This necessitates relatively large controlled areas in the valves, which usually conflicts with the requirements of small overall dimensions, quick response times and small control and static leakages in the injection system. In the prior art injection system of U.S. Pat. No. 6,189,517, the leakage problem is additionally exacerbated by the use of small area passages that connect the downstream side of the electrically actuated valves to the return line in order to eliminate possible uncontrolled injections and leakage of fuel past the closed nozzle into the engine which is possible because of the low viscosity of the DME. An improvement to this design has later been disclosed in which the electrically actuated valves are of a three-way spool type such that they can connect the injector nozzle alternately to the source of pressure and to the return line. Such a three-way spool valve partially solves the problem of parasitic leakage during the injection event that is present in the prior art system described in U.S. Pat. No. 6,189,517, but introduces leakages past the clearances in the spool valve. Such leakages have been found to negatively affect hydraulic efficiency and controllability even in the conventional diesel fuel injection systems. The approximately 10 times lower viscosity of DME would bring in an increase in leakage rate of about the same magnitude. Reducing clearances in the spool to limit the leakage is not technically feasible both because they are already close to a minimum and because low-viscosity fuels have poor lubricity, which greatly increases the risk of seizure in close-fit spools.
A possible solution to this is a three-way, hydraulically unbalanced valve with two tapered or flat seats, in which the pressurized volume can be isolated by relatively long precision-matched guide ensuring acceptable leakage. The basic design of such a valve is well-known in the art and is successfully utilized in the fuel injection systems to control relatively small pilot flows, as disclosed, for instance, in EP1120563A2. However, this principle can hardly be realized to control full flow of fuel to be injected, especially in the case of increased volume deliveries necessary when using DME, because the size of the valve becomes prohibitively large for the good response times and acceptable electrical power consumption to be maintained. Thus, a novel approach to the design of the hydraulic control system of the low-viscosity fuel common rail system is required.
The poor lubricity and potentially big leakage of the low-viscosity fuels, on one hand, and their low-sooting combustion properties, on the other hand, dictate the choice of relatively low injection pressures. Low fuel pressure levels not only require extra large flow areas in the control valves of the injection system, but also make it necessary to use high-flow nozzles in order to keep the injection time periods short enough for good engine fuel efficiency. The design of conventional high-pressure diesel injection nozzles which are used in the system disclosed in U.S. Pat. No. 6,189,517 B1, limits the size and number of injection orifices that can be used. Another unfavorable aspect of using conventional high-pressure diesel injection nozzles in a low-viscosity injection system is their relatively high cost and complexity.
Yet another negative aspect of using conventional diesel nozzles in a low-viscosity fuel system is that the leakage past the needle guide of such nozzles can also become relatively large. Therefore, it would be beneficial to design a low-viscosity fuel injection system that utilizes a nozzle which is characterized by reduced leakage and allows large opening flow areas. A reduction in the leakage can also allow cost reduction of the fuel system by making a fuel cooler redundant. The fuel cooler may otherwise be necessary because higher leakage causes conversion of more hydraulic power into heat which has to be removed from the system.
It is desirable to improve fuel economy of a low viscosity fuel-powered engine with a fuel injection system of the common rail type by means of limiting the parasitic hydraulic losses due to static and control fuel leakages, improving injection controllability and reducing duration of injection. It is also desirable to reduce the cost of the fuel injection system, which can be achieved by reducing leakages in the system and by simplification of the nozzle design.
The fuel injection system according to an aspect of the present invention incorporates a three-way electrically operated pilot valve that controls a hydraulically operated valve positioned between a common rail and a nozzle, a differential hydraulic valve positioned upstream of the nozzle with its outlet connected to the inlet of the nozzle, and an electrically operated, two-way, normally open spill valve positioned between the outlet of the hydraulically operated valve and a return line. The nozzle has a leak-free design with a poppet-type valve that is biased towards its closed position by a spring and can open outward in the direction of the engine combustion chamber when fuel pressure in the nozzle exceeds the nozzle opening pressure, which is determined by the backpressure in the combustion chamber of the engine and the force of the spring. Such nozzle design features significantly bigger, than in conventional high-pressure diesel nozzles, ratio of open effective flow area to valve lift.
All control valves in the present invention have either tapered or flat seats which provides for minimum possible leakage in the closed state of the valves. The pilot valve is very small because it only controls the little flow necessary for switching the hydraulically operated valve. Due to its small size, the pilot valve can be hydraulically unbalanced with positive sealing of both seats, and the leakage past the valve stem is also small due to its small diameter, typically 3 mm, and relatively long stem sealing length.
The hydraulically operated valve can also be made with relatively small stem diameter, typically 4 mm, because its lift can be relatively big compared to what is achievable with electrically operated valves. The sealing length of the stem can be made sufficiently long to achieve small leakage. Preferably, the hydraulically operated valve has tapered seat for positive sealing.
The spill valve can also be made very small, because its purpose is to assist the nozzle valve's and the hydraulically operated valve's closure and relieve residual pressure in the nozzle in order to limit possible leakage of the fuel into combustion chamber and/or uncontrolled injections that could otherwise be possible in case of imperfect sealing in the hydraulically operated valve and consequent pressure build-up in the nozzle.
The differential hydraulic valve performs the same function as that of the resilient check valve described in the U.S. Pat. No. 6,189,517 B1 referred to above. This function is to prevent the leakage of the fuel into the engine after the engine shutdown. The differential hydraulic valve stays fully open during engine operation and does not participate in the injection control.
Due to the design of the control system of the present invention, characterized by the fact that the electrically operated valves do not directly control the full flow of fuel to be injected into the engine, these valves and their actuators can be made sufficiently small to be fitted directly in the injector. This minimizes the dead hydraulic volumes and helps achieve more accurate control of fuel injection, in particular, of small fuel quantities.
Conventional high-pressure diesel injection nozzle can also be used in the present invention in cases when its injection spray pattern is more advantageous for a particular combustion system, than the spray pattern of the poppet-type nozzle. In such case the leak-off fuel from the low pressure side of the nozzle needle is connected either directly to the return line or to a backpressure regulator to achieve variable nozzle opening pressure.
Like reference numbers are used to designate corresponding parts of the systems depicted in the drawings.